Rear Brakes not working stopping only with front brakes

You have drum brakes in the rear, right? Probably the brake shoes are not broken in and working properly yet. If you didn't have the drums machined the process will take longer. If the adjustment is not right, maybe you'll need to remove the drums to clean the shoes and readjust them. If you didn't get the drums machined, you may want to do so at that time.